Control system



Oat.` 19 1948. R. R. THALNER 2,451,540

CONTROL SYSTEM Fiieduay 11, 1945 '5 sheets-sheet 1 Oct. 19 1948. RJR.THALNER 2,451,640

CONTROL SYSTEM Filed May 1l, 1945 5 Sheets-Sheet 2 VIDE 0 SIGNA/ AMI?OUTPU VIDEO AMR INVENTOR ROBE/er E. THM/VH? ATTORNEY Oct. 19 1948. R, R,THALNER f 2,451,640

CONTROL SYSTEM Filed May 11, 1945 5 sheets-sheet s SIGNAL OUTPUT wot-0 AMP VIDEO A11/IA mvENToR @035er 1Q. HAL/vfe svygw ATTORNEYV Oct. 19 1948.w R. R. THALNER CONTROL SYSTEM 5 Sheets-Sheet 4 Filed May 11, 1945INVENTOR- af 1Q. THALNER NNN Q5 u ATTORNEY Oct. 19 R R'THALNER 2,451,640

CONTROL SYSTEM Filed May 11, 1945 5 sheets-sheet '5` SIGNA/ OUTPUT V/PEOAMP ATTORNEY igatented ct.

rfisgili CONTROL SYSTEM Robert R. Thalner, Princeton, N. J., assigner tof America, a corporation Radio Corporation o of Delaware Application May11, 1945, Serial No. 593,153

14 Claims.

The present invention relates to a method and means for controlling thescanning beam intensity ina television camera tube.

In particular, the invention will be foundto be applicable to cameratubes of the so-called low-velocity scanning beam types. A tube known asthe Orthicon is thus .one of the types with which lthis invention findsparticular use. The Orthicon type camera tube is known, per se, in theart,V and details concerning its construction and general operation maybe had by reference to the article entitled The Orthicon, a televisionpick-up tube, by Messrs. Rose and Iams which was published in the RCAReview for October, 1939, and which appeared, With certain bibliography,in pages 186 to 199 inclusive.

The storage-type pickup tube known as the lOrthicon diiers from othercamera tubes used in television transmitting systems in the use of alow-velocity beam of electrons for scanning the mosaic. In an Iconoscopafor example, the cathode ray scanning beam impacts the mosaic element athigh velocity, and has the effect of exciting secondary emission fromthe mosaic surface. Variations in the number of these secondaryelectrons Which are collected constitute the video signal output, as isalready understood in the art.

While both of the above types of tube include a mosaic element on whichthere is set up an image in the .form of stored electrostatic charges,the V.principle of operation of the Orthicon is such that the electronsin the low-velocity scanningfbeamv may be collected from the beam as 4itpasses over the elemental areas of the mosaic surface and employed to.neutralize the positive charges existing .on these areas. The number ofscanning kbeam electrons collected at any instant will depend on themagnitude of the negative charge .deficiency possessed by the particularv surface area being instantaneously scanned by the cathode ray scanningbeam, and this negative charge deiiciency, in turn, in proportional tothe brilliance of the homologously associated point on the optical imageto be transmitted.

`Thus, the positive charges on the mosaic, which represent anelectrostatic image corresponding point-for-point. to the optical image,areneutralized by the scanning beam electrons. Due tothe differences inmagnitude of the positive charges acquired by the various elementalareas of the mosaic surface, the total number of electrons present inthe scanning beam is not always required to neutralize a particularmosaic area. "The excess beam electrons or, in

' tube.

other Words, those not collected by the mosaic, are returned toward theelectron gun end of the camera tube. Y

In one species of the proposed Orthicon camera tube, the video signaloutput is derived directlyfrom the mosaic as a result of variations inthe number of scanning beam electrons which are collected thereby. Inanother species, those electrons remaining in the beam after theelectrons therein required for yneutralization have been collected bythe mosaic are returned to an electron-collecting electrode adjacent theelectron gun and there collected to produce the signal current. In thelatter arrangement, the signal output current of the 4camera tube isequal to the original beam current as produced by the electron gun lessthe number of electrons lost by the beam to neutralize the positivecharges as it scans each point of the mosaic so that the quantity ofelectrons lost ,in this manner is proportional to the density of theelectron image on that particular mosaic portion and hence thebrilliance of the corresponding point on the optical image. Accordingly,the electron beam from the gunof the camera tube is modulated by themosaic potentials in accordance with the Characteristics of the opticalimage to be televised, and the modulated electron beam utilized as thevideo signal output of the tube.

'The number of .electrons instantaneously co1- lected from the vscanningbeam is controlled by the potential of the mosaic at the point ofinstantaneous scanning of the mosaic by the scanning cathode ray beamor, as above stated, on the negative charge deficiency of that elementalmosaic area. If such charge deficiency in zero, as `a result of zeroillumination being received from the corresponding point on the opticalimage, then .approximately all of the electrons present in the scanningbeam Yat that instant will be vreturned toward the electron gun. Thevnumber of electrons so returned Will be approximately .equal to fthenumber originally emitted, or in other Words vthe returned electronswill approximately equal in value the beam current of the Returnedelectrons approximately equal in number to those in the emitted scanningbeam Vwhen collected constitute one limit of video signalcurrent.

At the other extreme is the condition when substantially none of theelectrons emitted from the gun is returned from the moisaic. This occurswhen substantially all of the electrons present in the scanning beam arerequired to make up the negative charge deficiency on a particular 3mosaic area due to the extreme brightness of the corresponding orhomologous point on the optical image. The Zero current thus produced inthe output circuit constitutes the other video signal limit.

It will be clear from the above :description that under the idealconditions set forth lthe emitted scanning beam is modulated between thelimits of zero -and'100%. It will be further appreciated that to produce100% modulation substantially all of the scanning electrons must becollected by the mos-alc and none lall-owed 'to return toward theelectron gun end of the camera itube. In- -asmuch as the number ofelectrons in the scanning beam at any instant is norm-allysubstaniti'ally constant, it is obvious that `there is only one value ofnegative charge deficiency .at which any particular elemental mosaicarea will collect substantially all of the electrons present in thescanning beam and be completely neutralized thereby. This value ofnega-tive charge deiiciency at which substantially all of .the electronsin the scanning beam are collected to produce complete neutralizationrepresents the maximum value of brightness permissible at any one pointJon `the optical image without introducing distortion into the output ofthe ycamera tube.

The above statement will be appreciated when it is considered that anincrease in brightness beyond the maximum value mentioned will increasethe negative charge deciency on the Inos-a'ic element underconsideration to a point where there 4are insufficient electrons presentin the scanning beam to make up the deficiency. Distortion of thetelevised image accordingly results.

Since undermodulati-on of the scanning `beam of a low scanning beamvelocity camera .tube is undesirable, the beam current is usually set soythat 100% modulation will result at the maximum brightness levelanticipated for the particular object to be televised. It often happens,however, that for various reasons |the maximum brightness of the Iobjectexceeds this anticipated level, necessitating manual adjustment of theva'lue oi the beam current. On the other hand, should .the maximumbrightness of the object be below the anticipated level, thenoise-to-signal ratio in the output is increased unless manualadjustment is made. Y

In my copending U. S. patent application Serial No.572,009, filedJanuary 9, 1945, I have disclosed a method and mean-s'for maintainingthe beam current in a low scanning beam velocity television camera tubeof the type having a separate photocathodefully'rnodulated regardless ofvariations in the illumination, thereby producing a uniformly clean andpowerful signal with a minimum of shading and, in addition, eliminatingthe noise due to an unnecessarily 'high beam current=under low lightconditions. As shown in this copending application, the above isaccomplished by utilizing the secondary electrons emitted when theelectron image produced by lthe photocathode of the tube impinges on themosaic. Since the number of'secondary electrons so emitted isproportional to the average streng-th of the'overall illumination on thephotocathode, these secondary electrons are collected and .caused toproduce a voltage which is applied to the control electrode of thecamera tube gun so as to vary the'beam Y means for utilizing a por-tionof the video signal current of the tube in such a manner that the Y beam`current is of a value just suflicient to discharge the mosaic under thelight conditions-then prevailing. Y

The present invention features a method and output of the camera tube toaccomplish the same objectives, thus eliminating lthe necessity ofernploying 'a separate circuit for lcollecting and storing the secondaryelectr-ons emitted from the mosaic by the impingem'ent lthereon of theelec-tron image produced by the photocathode. Furthermore, sin-ce thepresent invention ldoes not require the presence of secondary electronsto crea-te a compensating control variation, the system disclosed hereinis not restricted to a camera tube of the type having la separatephotocath-ode. Instead, a camera tube may :be employed in which anobject to be televised is focused directly on a so-called -double'mosaicwhich, if desired, may be of the type disclosed by L. E. Flory in U. S.Patent No. 2,045,984, gran-ted June 30, Y1936, or, if the mosaic issemi-transparent, of the type shown in Paten-t No. 2,150,980, issuedMarch 21, 1939, .to H. G. Lubszynski et al.

One object of the present invention, therefore, is to provide a method.and means for automatically controlling the grid bias in a televisioncamera tube.

Another object of the invention is to provide a method and means formaintaining a substantially modulated beam current for a lowvelocityscanning beam type television camera 'tube regardless of variations inthe average overall illumination received from the object beingtelevised.

A further object ofthe invention is -to provide a method and. means forutilizing a portion of the video signal output of a low scanning beamvelocity `type television camera tube so as to control the beamintensity of theytube.

Other obje-cts and advantages will be apparent from the followingdescription of preferred forms of the invention and from the drawing, inwhich:

Fig. 1 illustrates schematically a preferred form of circuitincorporating the present invention;

Fig. 2 is a curve which is referred to in eX- plaining the operation ofthe circuit of Fig. 1; and

ing alternative larrangements in accordance with the present invention.

-Referring iirst to Fig. 1, there is shown a television pickup or cameratube I 0. TheV elements of tube I0 for producing an electrostatic imageare of the Ycharacter referred to in my abovementioned Ycopending U. S.patent'application, Serial No. 572,009. These elements hence are notherein to be described in detail, butthey will be understood to includea photocathode I2 on which an image of an object I4 is focused by meansof a lens I6. Photocathode I2 is connected to the negative terminal of abattery I8 or other source of potential. Illumination falling onphotocathode I2 causes an emission of electrons from the inner surfacethereof, such emission, as is well known in the art, being in the formof an electron image each point of which corresponds in density to thestrength of the illumination on the corresponding point of photocathodeI2.

The velocity of the electronsV thus emitted from the surface ofphotocathode I2 is increased by an accelerating electrode 20 which isshown as an annular band of metal onthe Ywall of tube I0 but which maybe of .any other suitable type, and which is connected toV anintermediate pointl on battery I8, toward a mosaic electrode 22 which isconnected to the positive terminal of battery I8. f While mosaic 22 ispreferably composed of semi-conducting material as set forth in a co-Figs. 3, 4, 5, and 6 are circuit diagrams showpending U. s. Vpatentapplication of Albert Rose, filed September 2.0,' 1940 as Serial No.357,543, now abandoned, it may, if desired, be of the so- Qalledd011b1m0Sa0 type as disclosed by Flory Patent, No. 2,045,984, abovementioned. The photocathodestructure I2 may readily be formed asdisclosed by Patent No. 2,248,977, granted to Flory et al. on July l5.,1941. A suitable electron lens (not shown) which may, for example, `beof the type disclosed in the mentioned Flory et al. Patent No.2,248,977, or in Patent No. 2,189,319, issued February 6, 1940, to G. A.Morton, is employed` to QGIlS on the mosaic electrode 22 the electronsemitted from the surface of photocathode l2.

Electrons impacting the mosaic 22., in turn, cause secondary electronsto be released therefrom, these secondary electrons being collected by ascreen 24. The release of secondaryfelectrons by a particular element orarea of mosaic 22 leaves such element with a positive charge or, inother words, with a negative charge deficiency. The amount of suchactual deciency is dependent uponthe density of the electron image atthat particular point.

The positively charged mosaic 22 is then scanned by means. of anelectron beam produced by an electron gun at the opposite end of tubeto., this electron gun being of any suitable type which includes acathode 26, a grid 23 andan accelerating anode (not shown). The beamdeflecting means of tube l0 is conventional and be magnetic,electrostatic or a combination. One preferred form may be considered asbeing of theform U. S. patenty application of Albert Rose, Serial No.357,543. The deflecting electrode system is consequently omittedr fromthe drawing for the sake. of clarity and simplicity of illustration.

As the scanning beam travels across the surface ot Inosaic22,Aelectrons; fromv the beam neutralize the positively lcharged mosaicelements. The beam normally supplies sufficient electrons., to make.v upthe negative charge deficiency of each image point or element. If aparticular-element isnot positively charged, or if such positive chargeissmall enough so that allV of the electrons. available inthe scanningbeam during the instant of passage are not required to make up. thenegative charge deciency on that element, then the remainingelectrons inthe beam or, in` other words,

those not. employed to neutralize. the electrostatic..

charge.: representing each image point or. element, are caused to returnalong a path substantially parallel with the scanning beam toward theend of,..tube, [0. from which they are emitted. Upon arriving atz theend of tube l0 containing the.

llltron gun, these returnedA electrons, are co1- lected by a signalplate 30 forming a part of. the tubev output circuit. Signal plate 30may be of any suitable design, such for example, as, a circular disl;having a central aperture through which the scanning beam electronsemitted from cathodel 25 may pass.

The signal output of tube I0 is amplified, if desirable or necessary, bytwo videor amplifiers 3 4gandA 36. In accordance with the presentinvention, a portion of the signal outputenergy of tube I is utilizedVvto provide a control variation for the electron scanning beam of thecamera. tube ill@ manner now to be described.

In, the cir-cuit shown in Fig. 1, the top plate (in. the, drawing) of acondenser 38 is connected between the two video` amplifiers 34. and 36.The.

other plateof condenser 38 is connectedto ground through a. resistor 40.Shunted across resistor disclosed by the aforementioned apart.

mis adiode 42 having its cathode connectedV to ground. A, second diode44 has its cathode connected to the anode of diode 42. The anode ofdiode 44 is connected to ground through a resistor 4.6 in parallel withaV condenser 48. Diodes 42 and 44 may, if desired, comprise the twosections of a tube of the general type known as the 6H6.

In the waveform 49 of Fig. 2 which is the waveform of a signal such asmight be present in the output. of amplifier 34, the video signalvariations are of such polarity as to extend in a negative direction toa certain maximum amplitude during each line-scanning interval, thismaximum amplitude being indicated by the reference character 50.Extending' in the opposite, or positive, direction from the A.-C. axis52 of the signal, are blankng pulses 54 formed by the application of aseriesl of blanking pulses 56 (Fig. l) from a suit,- able source (notshown) to screen 24 of tube l0 over conductor 5.8.

A port-ion of the signal output of video amplifier 34 such,l forexample, as represented by the Wave.- torm-4$i, is applied. to thediodes 42 and 44 through condenser 38. When a positive portion of wave49., such asI one of the blankin-g pulses, 54, reaches diodes 42 and 44,the former will be rendered conductive to. charge. condenser 38. by anamount determined by the height of the top of the` posi.- tive pulse 54above the A.C. axis 52.

Duringv the conduction of diode 4.2, both point 56 vand the cathode ofdiode 44 are at D.C/.l ground potential. When now a negative. portion ofwave. 49v reaches diodes 42 and 44, diode 44 willbe rendered conductiveto develop across the resistorcondenser combination 46, 4.8 in. theanode circuit of diode 44 a voltage which isequal to the amplitude ofthe mostv negative point 5i!` ofv wave 49. taken .withrespect to D..C`ground. Since this D.C. grou-nd level, as described above,A has. beenset at the top oi blanking pulse 54, the voltage` across. condenser 48will be that developed between point 59 and the top of. pulse' 54 or, inother words, will be the peak-to-peak voltage of the signal wave 49.

' When the.- illumination received by photocathode I2 from any point onobject I4 increases, the corresponding element of mosaic 22 becomes morepositive-a. This results in a decrease in the num.- ber of electronscollected by the signal plate 34 asthe electron beam. scans thatparticular mosaicelement, and as a result the most positive and negativepoints on the signal output curve, as,` for example, the point 50 andthe top of pulse 54Y on the illustrative waveworm 49, move. further YThis increase. in peak-to-peak signal. voltage increases proportionallythe charge on condenser 48..

The time constant of resistor 4S-and condenser 4.& is chosen to give asatisfactory D.C. variation with respect to changes in illumination,be-l 'ingpreferably in ther order of the time required tosscan severalframes. In any event, suchA time: constant should not be less than oneline-scan.- ning interval, as otherwise instantaneous' A.C. variationswill result which are objectionable-for reasons. which will later becomeapparent.

The` voltage-developed on condenser 48 is ap-y plied tothe control grid5810i a tube 60 which lss designed to act as a1D..-C. amplifier.inasmuch-Y as-the voltage developed on condenser 4B is negatire' withrespect. to. groundy an increase in the:

voltage oncondenserv 4% in response to arl-,increaseinthe peak-to-peaksignal voltagel output',

of camera tube. HJ'v places a more' negative bias?. onsthe. control grid58 ottube-lil). This reduces;

the plate current of tube 60 and causes the plate end of load resistor62 to become more positive. This decreased potential drop acrossresistor 62 is applied, by means to be 'hereinafter described, as adecrease in negative bias on the control grid 28 of camera tube i6 toincrease the intensity of the beam current of tube l@ to a point whereit is just suiiicient to neutralize the charges on mosaic 22 under theincreased illumination conditions and thus prevent overmodulation of thescanning beam.

Since tube t) is designed for substantially linear operation, variationsin the negative charge on condenser 48 will produce substantiallyproportional variations in the positive voltage appearing on the plateend of load resistor 62.

To obtain a control voltage for the grid 28 of camera tube l Which is ofcorrect polarity, the positive voltage appearing on the plate end ofload resistor 62 is bucked out with a negative voltage from a source ofpotential (not shown) connected to the terminal 54, this latter sourcebeing of such value as to produce a negative voltage at point 68 Whichvaries in substantially inverse proportion to the changes inY positivepotential appearing on the plate end of load resistor 62.

4'Ihe circuit of Fig. 3 is essentially the same as that of Fig. 1 withthe exception that two video amplifiers t8 and TQ have been added andthe D.C. amplifier tube 6i] and its associated elements omitted. Whereasin Fig. 1 the resistorcondenser combination 45, 48 was connected in theanode circuit of diode 44 to produce a negative charge on condenser 48,in Fig. 3 the same resistance-condenser combination is connected in thecathode circuit of diode 42 to produce a positive charge on condenser48. The varying .positive charge produced on condenser 48 duringoperation of the system causes variations in the negative D.C. biasplaced ongrid 28 of camera tube lll by a battery or other source ofpotential 'I2 through resistor 46.

While the systems of Figs. 1 and 3 are satisfactory for many operations,they do not have the same stability as the circuit of Fig. 4 (later toVbe described), clue to the fact that they are regenerative. If the biason the camera tube IG is accidentally set too positive, the scanningbeam current is caused to reach an unnecessarily highV level; Theexcessively large number of electrons present in the scanning beam tendsto produce a condition known as shading, which has the same eiect,insofar as the output of the camera tube is concerned, as an increase inthe amount of illumination falling upon the photocathode. Consequentlythe output of the camera tube increases, which in turn causes a stillfurther increase in the scanning beam current.

In Fig. fi is shown a system whereby the regenerative operation of thecircuits of Figs. 1 and 3 is overcome by the utilization of a limitertube 'I4 connected across the condenser 8. Whenever the positive voltageon condens-erw increases to a point where it exceeds a predeterminedYmaximum value as determined by the potential of battery T6, diode 'I4is rendered conductive.l The voltage developed on condenser 48 isthusprevented from reaching a point suliciently high to cause objectionableregeneration.

In Fig. 5 is shown means whereby an automatic video gainV control may beadded to the system of Fig. 4. This is accomplished by utilizing aVparallel resistor-condenser combination 18, 80 in the anode circuit ofdiode 44. A negative voltage on condenser sfwill-be developed duringoperation of the system of Fig. 5 in the same manner that a negativevoltage is developed on condenser 48 in the system of Fig. 1.

If the signal fromrcamera tube l0 increases, forY example, there isYneed for more scanning beam current and less video gain. In Fig. 5 theVoltage on condenser 48 Will become more positive due to the increase inpeak-to-peak signal voltage as hereinabove brought out, and thisincrease in voltage on condenser 48 is applied to grid 28 of camera tubeIll, as in Fig. 4, to increase the scanning beam current. The voltage oncondenser et, however, will become more negative, and this increase innegative voltage on condenser e@ is applied over a conductor 82 toreduce the gain of the video amplifier 36in any suitable manner, such asby increasing its negative bias.

The system of Fig. 6 is similar to that of Fig. 5, except that the gaincontrol voltage is appliedto the signal circuit not only over conductor22Y after a portion of the signal has been detected by diodes 4.2 and44, but also over a con-` Vductor 8e before the signal is detected. Thistends t0 stabilize the system because it eiectively reduces the gain ofthe rectifier circuit.l

If the voltage on condenser d8 exceeds a predetermined maximum value,diode T4 is rendered conductive, thereby placing all ofthe voltage thatwould appear on. condenser 48 across condenser il. This increasednegative charge on condenser 3Q is applied over conductors 82 and 8d toreduce the amplitude of the signal and restore the normal operatingstatus of the system. If desired, the limiter tube 'M and battery 'l5may be omitted from the circuit of Fig. 6. Such an omission, hovvever,`tends for increased instability and possibly excessive regeneration.

Other modications, of course, will be obvious to those skilled in theart to which the inven-l tion is directed. Y

Having now described the invention, what is claimed and desired to besecured by Letters Patent is the following: f 1. In a television system,a camera tube of the type in Which a mosaic electrode Within said tubeis adapted tobe scanned by a low-velocity scanning beam to therebydevelop an output rectified energy toV control the degree of amplicationof said signal in inverse proportion thereto.V l Y 2. In a televisionsystem, Va camera tube in which a cathode ray scanning beam is developedto scan a mosaic electrode on which electrostatic charges are developedin proportion to the brillance of an optical image, the signal output ofsaidtube having a peak-to-peak value which is a measure of thebrilliance of said optical image, a peak-to-peak rectifier, means forapplying the signal output of said camera tube to said peak-to-peakrectifier, an energy storagel circuit, means for applying the output ofsaid peak-to-peak rectifier to said energy storage circuit so as todevelop a relatively smooth D.C`.

' having an' amplitude approximately equal to the peak-to-peak value ofthe signal output of said camera, tube, and means for applying theoutput of said energy storage circuit to said camera 9 tube to controlthe intensity of said scanning beam in direct proportion to saidpeak-to-peak value of the signal output.

3. A television system in accordance with claim 2, in which the saidenergy-storage circuit comprises a resistance-condenser combinationhaving a time constant equal at least to the time required for the saidscanning beam to make one complete scansion of the said mosaicelectrode.

4. In a television system, a camera tube of the type in which a mosaicelectrode within said tube is adapted to be scanned by a low-velocityscanning beam to thereby develop output signals, a pair of videoamplifiers in the output circuit of said camera tube, a pair of diodes,means for applying a portion of the signal output of said camera tubeappearing between said video amplifiers to the anode of one of saiddiodes and to the cathode of the other of said diodes, means connectingthe cathode of said one diode to ground, a parallel resistor-condensercombination, means connecting the anode of said other diode to groundthrough said parallel resistorcondenser combination, and means forapplying the charge developed on said condenser during Aoperation of thesystem to control the intensity of said scanning beam.

5. The combination of claim 4 in which said last-mentioned meansincludes a D.C. ampli- Iier, the charge developed on said condenserbeing applied to control the Vvoperation of said D.-C. amplifier.

6. The combination of claim 2, further comprising means for limitingapproximately a predetermined value the energy stored by saidenergy-storage circuit.

7. The combination of claim 2, further comprising means for limiting toapproximately a predetermined value the energy stored by saidenergy-storage circuit, said limiting means comprising a diode and asource of potential con-.

nected in series across said energy-storage circuit, the positiveterminal of said potential source being connected to the cathode of saiddiode and the anode of said diode being connected to the positive outputterminal of said energy-storage circuit.

8. In a television system, a camera tube of the type having a mosaicelectrode, and wherein a low-velocity scanning beam is developed anddeeoted to Yscan said mosaic electrode to thereby produce outputsignals, means for amplifying th'e signal output of said camera tube, apair of diodes, means for applying a portion of the signal output ofsaid camera tube to the anode of one of said diodes and to the cathodeof the other of said diodes, a first energy-storage circuit connectedbetween the cathode of said one diode and ground, a vsecondenergy-storage circuit connected between the anode of said other diodeand ground,

means for applying the output of .said first energy-storage circuit tocontrol the intensity of said scanning beam, and means for applying theoutput of said second energy-storage circuit to control said amplifyingmeans.

9. In a television system, a camera tube of the type wherein acathode-ray scanning lbeam is developed and then deflected to scan amosaic electrode in said tube to produce output signals, a firstamplifier and a second ampliiier connected in cascade, means forapplying th'e signal output of said camera tube to said first amplifier,a pair of diodes, means for applying a portion of the signal output ofsaid first amplier to the anode of one of said diodes and to the cathodeof the other of said diodes, a rst energy-storage circuit connectedbetween the cathode of said one diode and ground, a secondenergy-storage circuit connected between the anode of said other diodeand ground, means for applying the output of said first energy-storagecircuit to control the intensity of said scanning beam, and means forapplying the output of said second energy-storage circuit to control thegain of both said first and second ampliers.

10. In a television system having a camera tube of the low-Velocityscanning-beam type, the method for automatically adjusting the electronbeam current to light intensities which comprises, deriving from saidtube a picture signal representing a succession of instantaneous valuesof light received by said tube, rectifying a portion of said picturesignal and deriving therefrom a unidirectional signal proportional tothe peak-to-l peak amplitude of said picture signal, averaging saidunidirectional signal over a plurality of scanned elds, and using saidaveraged unidirectional signal to control the electron beam current indirect proportion to said averaged unidirectional signal.

11. The method according to claim 10 including the steps of amplifyingsaid picture signal and using said averaged unidirectional signal tocontrol the amplication in inverse proportion to said averagedunidirectional signal.

12. The method according to claim 10 including the steps of amplifyingsaid picture signal both' before and after performing said rectifyingstep, and using said averaged unidirectional signal to control bothamplirlcations in inverse proportion to said averagedundirectionalsignal.

13. In a television system, a camera tube of the low-velocityscanning-beam type, said tube containing means forcontrolling theelectron current of the beam and means providing a picturesignal output,means for rectifying a portion of said output and deriving therefrom aunidirectional potential proportional to the peak-to-peak amplitude ofsaid output, means for averaging said unidirectional potential over aplurality of scanned fields, and means for applying said averagedunidirectional potential to said beam control means in a direction tovary the electron current in direct proportion to said peak-to-peakamplitude.

14. The combination dened by claim 13 wherein said applying meanscomprises means for placing Aan upper limit upon the unidirectionalpotential applied to said beam control means.

ROBERT R. THALNER.

REFERENCES CITED The following references are of record in the le ofthis patent:

UNITED STATES PATENTS Number Name Date 2,084,700 Ogloblinsky June 22,1937 2,182,578 Blumlein Dec. 5, 1939 2,222,759 Burnside Nov. 26, 19402,292,817 Bedford Aug. 11, 1942 2,307,375 Blumlein Jan.v 5, 19432,345,282 Morton Mar. 28, 1944 2,404,098 Schade July 16, 1946 2,404,173Hansen July 16, 1946

